1. Field of the Invention
This invention relates to a continuous process for producing long .alpha.-alumina fibers.
Because of their excellent properties, alumina fibers hold an important position in applications including various heat insulating materials, reinforcements for composite materials, and the like. Among others, long continuous fibers can take a variety of forms such as ropes, nets, woven fabrics and the like. Accordingly, they are advantageous in that they can be used in the form most suitable for the intended purpose so as to exhibit their full performance and in that they are easy to handle. However, long continuous fibers are hard to produce, as compared with short fibers. Thus, there is a demand for an industrially advantageous process for producing long continuous alumina fibers.
The .alpha.-alumina fibers produced by the process of the present invention exhibit excellent heat resistance at high temperatures above 1,600.degree. C. and have high tensile strength and high elastic modulus, so that they are useful as reinforcements for metals and ceramics and as structural materials for high-temperature furnaces.
2. Description of the Prior Art
Conventionally, long alumina fibers have been produced by spinning a slurry composed of alumina powder and an aqueous solution of an alumina precursor and firing the spun fiber. Such processes are disclosed in U.S. Pat. Nos. 3,808,015 and 3,953,561. The former relates to a spinnable composition comprising specific small-sized particles containing at least 80% by weight of alumina, and an aqueous solution of an alumina precursor. The description given therein is concerned chiefly with the particle diameter distribution of the small-sized particles, and an additional mention is made of a process for preparing fibers by using the aforesaid composition. The latter relates to a process for producing continuous polycrystalline alumina yarns. According to this process, solid particles containing at least 80% by weight of alumina are mixed in an aqueous phase having an alumina precursor dissolved therein, the ingredients being used in such proportions that the solid particles provide 13 to 97% by weight of the total available oxides in the mixture. This mixture is extruded through a spinneret having a plurality of orifices to form a multifilament yarn containing 30 to 60% by weight of volatile materials. With consideration for the shrinkage of the yarn in subsequent steps, the yarn is wound on a collapsible bobbin to form a yarn package, which is heated (or prefired) at a temperature of 300.degree. to 1,400.degree. C. until the volatiles content of the yarn is reduced to 11% by weight or less. Thereafter, the yarn is unwound from the yarn package and heated to an apparent temperature of 1,000.degree. to 1,800.degree. C.
In these slurry spinning processes, it is necessary to fire the precursor fiber at a high temperature of 1,000.degree. to 1,800.degree. C. Generally, the precursor fiber is liable to breakage in this firing step, because of rapid shrinkage caused by evaporation of volatile components. For this reason, it has been believed that it is difficult to heat the precursor fiber to 1,000.degree. C. or above at a stroke and a prefiring step is required to heat the precursor fiber gently at a rate of not greater than 20.degree. C. per minute. Owing to the lack of balance between the high spinning rate and the low prefiring rate, it has been difficult to prefire the spun precursor fiber continuously.
Accordingly, as described in the above-referenced U.S. Patents, it has been common practice to wind the spun precursor fiber on a bobbin and prefire it gently in a batchwise manner. Alternatively, it has also been known to form large amounts of precursor fibers into bundles or mats and prefire them gently on a belt conveyor. Where these techniques are employed in the production of silica free alumina fibers, the resulting prefired fiber is very fragile and, therefore, may be damaged during unwinding prior to high-temperature treatment at 1,000.degree. C. or above. This brings about a deterioration of fiber properties and makes it impossible to obtain an alumina fiber of good quality.